Managing Information Technology

Chapter 3 • Telecommunications and Networking 79

line into two types of communication channels. The B, or
bearer, channel transmits voice or data at rates of 64 kbps,
faster than is possible using a modem. The D, or data,
channel is used to send signal information to control the B
channels and to carry packet-switched digital data.
Two narrowband ISDN services are offered. The
basic rate offers two B channels and one 16-kbps D channel
(a total data rate of 144-kbps) over a single twisted pair.
Each basic rate line is capable of supporting two voice
devices and six data devices, any two of which can be
operating simultaneously. The primary rate provides
23 B channels and one 64-kbps D channel (for a total
data rate of 1.544 mbps) over two twisted pairs.
Although not widely available, broadband ISDN offers
data transmission rates of over 150 mbps. Therefore,
ISDN provides a significant increase in capacity while
still using the public telephone network.
The developments in ISDN are a part of the digitiza-
tion of the public telephone network. However, ISDN has
never caught on in a big way, and it now seems destined to
be bypassed by other digital developments such as DSL (to
be covered in a later section) and IP (Internet Protocol)
telephony. At present, ISDN service is available on many
telephone lines in the United States, but it is relatively
expensive compared to other options such as DSL.

Packet-Switched Networks. Packet-switched net-
worksare quite different from the switched-circuit (DDD
and WATS, ISDN) and dedicated-circuit (leased lines,
satellite) networks previously described. In switched- and
dedicated-circuit networks, a circuit is established between
the two computers that are communicating, and no other
devices can use that circuit for the duration of the connection.

By contrast, a packet-switched network permits multiple

connections to exist simultaneously over the same physical

circuit.Packet switchingis a store-and-forward data

transmission technique. Communications are sent over the

common carrier network, divided into packets of some

fixed length, perhaps 300 characters (see Figure 3.9).

Control information is attached to the front and rear of this

packet, and it is sent over a communications line in a single

bundle. Packet switching is quite different from usual

voice and data communications, where the entire end-to-end

circuit is tied up for the duration of the session. With packet

switching, the network is used more efficiently because

packets from various users can be interspersed with

one another. The computers controlling the network will

route each individual packet along the appropriate path.

Apacket assembly/disassembly device(PAD) is

used to connect the user organization’s internal networks

(at each of its locations) to the common carrier network.

The organization must, of course, pay a fee to make use of

the common carrier network. In some cases, the user

organization provides the PADs and pays a fixed fee for a

connection into the common carrier network plus a charge

for the number of packets transmitted. In other cases, the

user organization might contract with a firm that manages

and operates the entire WAN for the user organization,

including the PADs needed at each location. This

contracting-out practice used to be called a value added

network (VAN), but that terminology has largely disap-

peared. Today such a packet-switched WAN is usually

called a managed network. In the United States, managed

network services are available from AT&T, Sprint Nextel,

Verizon, and other providers. Packet-switched networks

are quite common, including corporate networks that

Common

Carrier

Network

Backbone

Network

Backbone

Network

Backbone

Network

Backbone

Network

Backbone

Network

PAD

PAD

PAD

PAD PAD

Key: PAD  Packet

assembly/disassembly

device

FIGURE 3.9 Packet-Switched Network Connecting Five Organizational Locations